It is required to carry out the direct flaming of steels in the heating zones without causing oxidation.
Conventionally known burners of this type are a high speed jet burner which directs flames against the steel strip and heat it by convention heat conduction, and on the other hand a radiant cup burner which heats an inner surface of a burner tile at high temperatures for heating the strip by radiant heat conduction therefrom.
The high speed jet burner burns mixture gas in a combustion chamber and jets out a combustion gas at high speed from a throttled nozzle. This burner uses a flow flux of high temperatures in a range of relatively low temperature of the heat material. However, since the flame during combustion reaction directly collides against the strip, slight oxidation is inevitably caused due to O.sub.2, O, OH and others existing therein.
The radiant cup burner rapidly burns a mixture of the air and fuel gas, which were mixed in advance in a hemi-spherical cup of the burner tile for providing rapid combustion reaction, so as to increase temperature of the inner surface of the burner tile, and heats the strip by the radiant heat conduction from the inner surface. This burner uses a flow flux of high temperatures in a range of high temperature of the heat material. If the fuel gas is burnt at the air ratio of not more than 1.0, it is possible to introduce reducing non-burnt contents such as CO, H.sub.2 and others in the combustion gas, and if this combustion gas contacts the strip, it is possible to effect heating without causing oxidation but causing reduction.
Thus, the radiant burner is suitable for heating without causing oxidation. But, since this is of the pre-mixture type system and it is harmful to previously mix air, which is pre-heated at the high temperature, in the combustion gas, the combustion air can not be preheated. Therefore, sensible heat of an exhaust gas by pre-heating the air can not be obtained, and so an independent means should be provided for yielding the sensible heat of the exhaust gas to save energy. It is useful to preheat the air for increasing the flame temperature, and it is effective to reduction by CO, H.sub.2 to increase the flame temperature. Accordingly, it is not preferable in view of the heating without oxidation not to preheat the air. In addition, a provision of a premixture device or a counter-flame checking device causes high costs of equipment.
Further, this kind of burner cannot be used with preheated combustion air, heating without oxidation is limited to a temperature of 750.degree. C., and if heating is required at higher temperatures, this burner is not applicable.
For solving such problems involved with the prior art, there have been proposed Japanese Application Laid Open No. 58-107,425 and Japanese Application Laid Open No. 60-26,212. These burners are defined with a plurality of combustion air jetting outlets in a space circumferentially of an inner wall of a tubular burner tile having an open end, and with fuel gas jetting outlets centrally of the burner tile, and the combustion air jetting outlet is formed in such a manner that the air jetting direction has an angle of not more than 60.degree. with respect to a tangent of the inner circumference of the burner tile. This burner does not require the pre-mixture of the combustion gas and the air, and can heat the strip effeciently. Unfortunately this burner has problems in that the range of the flame is unstable and narrow where the strip is heated without causing oxidation, and is not practical for use in a production line.
In view of these circumstances, it is an object of the invention to provide an improved burner of this kind which eliminates such defects of the prior art. The present invention comprises a burner for directly flaming steel materials for reduction without causing oxidation.
It is another object of the invention to provide a burner form direct flaming for reduction which can use preheated air.